Summary In order to understand the principles of long-range connectivity in cortical communication, our efforts have focused on the following two projects. Project 1: Functional connectivity of diverse long-range inputs to the primary somatosensory cortex. To achieve a mechanistic understanding of the functional connectivity among different cortical areas, we have systematically examined the synaptic strength from different brain areas to diverse neuronal types in the primary somatosensory cortex, using electrophysiological methods combined with optogenetic tools and viral tracing methods. Our results suggest that sensory-related feedback information is transmitted to the primary sensory cortex by engaging PV (parvalbumin) GABAergic interneuron-mediated feedforward inhibition, while motor-related feedback information is propagated to S1 through VIP (vasoactive intestinal peptide) GABAergic interneuron-mediated disinhibition. Thus, primary sensory cortex may parse information from diverse feedback projections by means of input area-dependent, preferential recruitment of specific types of GABAergic interneurons. Project 2: Circuit basis of functional diversity in pyramidal neurons of the primary somatosensory cortex The associative layer of cortex is the main cortical layer for cortical interaction. Neuronal activity in the associative layer of sensory cortex is highly heterogenous. The diverse subnetworks in the associative layer show correlated neuronal activity in relation to various aspects of the animals behavior. We hypothesize that dedicated streams of information impinge onto functionally relevant subnetworks, and these distinct subnetworks broadcast their information to different brain regions. To understand the circuit mechanism of functionally relevant subnetworks in sensory cortex, we have examined the activity of large neuronal populations together with multiple behavioral variables, using two-photon calcium imaging, optogenetics, anatomical tracing, and monosynaptic tracing methods. We are currently examining whether functionally relevant subnetworks are organized by their long-range projection targets and constrained by specific long-range presynaptic networks and local GABAergic interneurons.